mehargags wrote:I still do not understand your config and work flow. What for and how are you using turbo SMTP. Please try explain in more clearer way.

I have a vps with an installed domain with vestacp.
I have 20 email accounts on this domain.
on the individual mail clients I installed turbosmtp.
When I send one of these mail accounts to another account, these emails are blocked by my vps.

Seems you messed up with your MX records. If im not wrong..
Is it your vesta domain dns settings > MX entries pointing to its own domain name ?
If yes, it might spam check blocked due to spoofed email. because, your outlook sending from (a@domain.com) to (b@domain.com) but via turbo SMTP.
While it recved by vesta mx, it thinks it seems spoofed email.